Field of the Invention and Related Art Statement
The present invention relates to a voltage non-linear ceramic resistor composed mainly of zinc oxide. More particularly, the invention relates to a method of manufacturing a voltage non-linear resistor to be used in overvoltage-protecting devices such as lightning arrestors, and also relates to a highly densified voltage non-linear resistor.
Since the voltage non-linear resistors composed mainly of zinc oxide have excellent non-linear voltage-current characteristics, they are widely used in lightning arrestors and surge absorbers to stabilize the voltage and to absorb surges. In case of manufacturing the voltage non-linear resistor, a small amount of an oxide or oxides of bismuth, antimony, cobalt and/or manganese, which serve as a substance for introducing the voltage non-linearity in the sintered body, is mixed with zinc oxide which serves as the main component, and then the mixture is granulated and shaped into a desired configuration. The shaped body is then subjected to a sintering process. In a preferred case, an inorganic material is applied on a side surface of the sintered body and, thereafter the assembly is subjected to a secondary sintering process, to form a high resistance layer. Electrodes made of aluminum, for example, are then applied on opposite surfaces of the finally sintered body. In order to use the thus obtained voltage non-linear resistor in the lightning arrestor in which very large surges have to be absorbed, it is desirable to make the surge withstanding capability of the voltage non-linear resistor as large as possible. The surge withstanding capability of the voltage non-linear resistor may be represented by the maximum electric current value at which the resistor is not broken down or a flashover does not occur under the application of an impulse electric current having a waveshape of 4/10 microseconds two times for each five minutes and stepping up the electric current value.
It is considered that the value of surge withstanding capability of the voltage non-linear resistor depends on the amount and diameter of voids existing in the sintered body. That is to say, it is considered that when applying the 4/10 .mu.s impulse electric current to the voltage non-linear resistor, the destruction of the resistor is caused by thermal stress. Therefore, if the mechanical strength of the sintered body is made high by decreasing the voids in the sintered body, it is expected that the surge withstanding capability thereof would be improved, since the electric current is likely to be concentrated at the tip of the void. If such local concentration of electric current occurs at the tip of the void, the temperature at the tip of the void is locally increased, because the heat conduction of the sintered body surrounding the void is small under applying the electric current for only a short time such as 4/10 .mu.s. If the thermal stress generated by this temperature increase becomes more than the mechanical strength of the sintered body, the resistor would be broken. Therefore, it is necessary to make the mechanical strength of the sintered body high and to remove the voids for the purpose that local concentration of electric current will not likely occur.
Further, in order to effectively prevent an electric discharge caused by the flashover of the voltage non-linear resistor, it is necessary to improve the coherency of the high resistor layer onto the side surface of the sintered body.
Japanese Patent Laid-open Publication, Kokai Sho No. 58-28,802 discloses a method of reducing the voids in voltage non-linear resistors, in which the shaped body is heated up to 1,300.degree. C. and during this heating cycle, the sintering is carried out under a reduced pressure lower than the atomspheric pressure within a temperature range from 800.degree. C. to 1,150.degree. C. In this publication, it is only indicated that the surge withstanding capability under the application of 2 ms rectangular electric current is improved, but there is no indication of the characteristic with respect to the surge withstanding capability under the application a 4/10 .mu.s impulse electric current. In a case where the 2 ms rectangular electric current is applied to the voltage non-linear resistor and the value thereof is stepped up until the resistor is broken, the feedthrough breakdown would occur in the resistor. On the other hand, in case of applying the 4/10 .mu.s impulse electric current, the feedthrough breakdown is not generated, but the parting breakdown would occur. Therefore, it is considered that the voids existing in the sintered body act in different manners on the surge withstanding capability for the 2 ms rectangular electric current and 4/10 .mu.s impulse electric current. The feedthrough breakdown is a breakdown such that a hole having a diameter of about 1 mm is formed through the voltage non-linear resistor and thus the resistance thereof becomes 1 k.OMEGA. or less so that the non-linear voltage current characteristic is substantially removed. The parting breakdown is a breakdown by which the voltage non-linear resistor is cracked or crushed and is broken into many pieces. As explained above, it is considered that the parting breakdown is attributable to the thermal stress generated in the sintered body when the impulse electric current is applied thereto.
Also, in the method disclosed in the Japanese Patent Laid-open Publication Kokai Sho No. 58-28,802, the shaped body is sintered under the reduced pressure until the sintering temperature becomes 1,150.degree. C., so that the added component or components as an additive are vaporized and the uniformly sintered body can not be obtained. Additionally the oxidation of the sintered body is started when the sintering temperature becomes over 1,150.degree. C. Therefore, if the shaped body has a large dimension such as 47 mm in diameter, 25 mm thickness, oxidation is not effected sufficiently into the center of the body, so that the non-linear voltage current characteristics which are the same as that of a resistor sintered under normal pressure can not be obtained. If the sintering time is extended in order that sufficient oxidation is effected inside the sintered body, zinc oxide grains are grown excessively in the sintered body, so that the threshold voltage (V.sub.lmA/mm) at which the resistor begins to show the non-linear voltage-current characteristic becomes unfavorably low. This threshold voltage (V.sub.lmA/mm) is a voltage at which the non-linear voltage current characteristic appears, and may be defined as a voltage appearing across unit thickness viewed in the direction of the electric current when the electric current of 1 mA is supplied to the resistor under the application.
As a measure for restraining the evaporation of the added component during sintering under reduced pressure, it is suggested that the shaped body is buried in powders including the relevant component and is then sintered. In this case, if the sintering temperature is increased until the sintered body is densified, the powders are adhered or applied to the sintered body so strongly that the side surface of the body is not smooth.
Furthermore, it is necessary to form high resistance layers on the side surfaces of voltage non-linear resistors to be used in the over-voltage protecting devices such as normal lightning arrestors in order to effectively prevent flashover along the side surface. The resistance layer is usually formed by applying an inorganic material layer on the side surface of the body to be sintered, and reacting the inorganic material with the material constituting said surface by sintering the body. Therefore, it is very important that the inorganic material applied on the surface is not separated therefrom during the sintering. In the known method disclosed in Japanese Patent Publication Kokai Sho No. 58-28,802, the coherency between the body to be sintered and the inorganic material is small because the body to which the inorganic material should be applied is a shaped body or a degreased body. Also, since the body to be sintered suddenly shrinks at a sintering temperature of about 850.degree. C., there is a so large difference in the shrinkage between the inorganic material and the shaped body to be sintered, and thus, the inorganic material peels from the body. Thus, there is a drawback in the conventional art that the high resistance layer can not be formed firmly and uniformly on the side surface of the voltage non-linear resistor.